Rotomolding is used for the manufacture of simple to complex, hollow plastic articles. It can be used to mold a variety of materials such as polyethylene, polypropylene, polycarbonate, polyamide, fluoropolymers, polyvinyl chloride (PVC) and mixtures thereof. Linear low density polyethylene (LLDPE) is often used as disclosed for example in “Some new results on rotational moulding of metallocene polyethylenes” by D. Annechini, E. Takacs and J. Vlachopoulos in ANTEC, vol. 1, 2001 and in U.S. patent application 20050255264.
It is often desirable to form a multi-layer rotationally molded article, to combine the attributes of two different polymeric materials. Presently, there are two methods used commercially to produce a multi-layer article—both involve a manual addition of the second layer components after the first layer has formed. The first involves the use of an insulated drop-box that is attached to the mold to keep one powder cool, while the first layer is formed in the mold, and at the proper time to open a valve that allows this powder to enter the hot mold and then be rotated to form a second layer. The second method involves a process whereby the mold is removed from the oven and a second layer material is added manually to either a hot or a cold mold, followed by rotationally molding the second layer, such as found in U.S. Pat. No. 4,988,549 (polyamide layer and polyisocyanate/polyepoxide layer) or WO 2005/115753 (fluoropolymer/non-fluoropolymer such as polyolefins). Unfortunately, both of these methods result in lowering the productivity of the molding operation, either due to fewer molds being placed on a given arm, or due to the added time for “two” rotational molding cycles. There is a need for a one-shot process for efficiently producing multi-layer rotationally molded articles.
Several one-shot processes have been used to prepare a multi-layer rotomolded articles. WO 2004/045849 describes forming a multi-layer article by a one-shot process, based on the observation that the smallest diameter particles reach the walls of the mold fastest. By using the outer material of small particle size and an inner material of larger particles size, it could be possible to prepare a two-layer rotomolded article. The reference also describes using a mixture of an outer layer powder having a melting point of less than 145° C. and an inner layer powder having a melting point of greater than 145° C. The '849 reference also describes combining these methods by using a blend of a lower particle size, lower melting point inner powder with a higher particle size, higher melting point outer layer. Unfortunately, this method does not produce a clean layer separation, producing a fair amount of overlap and no well defined separation of the layers. If the second layer is formed by delaying the melting of the second layer powder, a rough inner surface is formed. JP 48-74741 describes the use of a low density polyethylene (LDPE) (M.P. 110° C.) to contain polyamide (PA) 6.6 powder (Tm=260 C) with a 103° C. HDPE outer layer. This leads to release of the PA powder before the PE layer is fully formed, resulting in poor layer separation, the PA layer being filled with PE particles resulting in poor barrier performance and poorer physical properties since these materials are not compatible. The adhesion between layers is purely mechanical and no chemical bonding between the layers is described.
There is a need for a single-step rotomolding process for producing a multi-layer molded article which provides for a strong chemical bond between layers without producing a rough inner surface.
Bag materials have been used to produce two layer tank using the same polymer, such as a tank using: (1) PE for both inside and outside layer with two different colors; or (2) a tank with a solid outer layer of PE and a foamed inside skin of PE. In both these cases a PE bag is used to contain the inside layer and there is no concern about mechanical integrity or barrier performance since both layers are formed with the same polymer.
Surprisingly it has been found that an article having two or more separate layers of different polymers having different melting points can be formed in a rotational molding process by a one step process, through the use of an outer layer polymer powder separated from an inner layer polymer powder by a release film. High quality moldings with both superior layer separation and strong adhesion between the two layers of different chemical composition are formed when the a release film having the proper composition and melting point is combined with the proper inner layer material and outer layer materials. The layer separation is needed for superior barrier performance or chemical resistance, while the adhesion between layers is needed to insure a high quality tank or container. The release film may form an enclosure or bag containing the inner layer polymer powder—or it may serve as a barrier between an insulated cavity in the mold or as a release mechanism for a drop-box. The article formed by the process of the invention has excellent layer separation, maximizing the mechanical and barrier properties of each layer while providing strong interlayer adhesion due to chemical compatibility or chemical reactivity between the layers.